ci: run expensive tests on push builds to integration branches

ci: run expensive tests on push builds to integration branches

Derrick Stolee suggested [1] that expensive tests should be run at a
regular cadence rather than on every PR iteration. Gate GIT_TEST_LONG
on push builds to the integration branches (next, master, main, maint)
so that the EXPENSIVE prereq is satisfied there but not during PR
validation, where the extra minutes of wall-clock time do not justify
themselves.

[1] https://lore.kernel.org/git/e1e8837f-7374-4079-ba87-ab95dd156e33@gmail.com/

Helped-by: Derrick Stolee <derrickstolee@github.com>
Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>